This invention relates to apparatus for detecting jams in a document path and, in particular, relates to such apparatus for use in a document path associated with a high speed printer in conjunction with a data processing system.
Presently known jam detectors have not been entirely satisfactory when applied to printer devices operating with data processing systems since these printer devices are operating at higher and higher speeds and reliable paper flow becomes more critical at higher speeds. One form of prior art jam detector utilizes a resettable timing device having a timing cycle of predetermined duration. The timing control is responsive to the trailing edge of the sheet entering a processing station to reset the timing device. If the timing device is not reset within a predetermined time, then a jam condition is indicated. This type of jam detector proved to be unsatisfactory when a series of documents being fed along a document path experienced a temporary slowdown or partial jam, possibly caused by one document being fed improperly. So long as any gap remained between the sheets, the trailing edge was sensed and the timing device was reset. By the time a jam was sensed, several documents could be involved and this resulted in loss of data due to tearing of sheets backed up along the document path. Any loss of data due to damaged documents is considered unacceptable. An attempt to reduce the duration of the timing cycle resulted in a number of unnecessary jam indications since many of the cases of temporary slowdowns or partial jams cleared themselves so no real jam ever occurred. Any unnecessary lost time in a data processing system is likewise considered unacceptable.
Another type of prior art jam detector utilizes a number of sensors along a document path and the progress of the documents along the path is monitored. By considering the expected progress of the document along the path, control circuits are provided to signal a jam condition whenever the detected progress does not correspond to the expected progress. This prior art system proved unsatisfactory since it was not possible to accurately define the time for arrival at points along the sheet path with sufficient accuracy to reliably sense jams in the presence of temporary slowdowns and partial jams as discussed above.